1. Field of the Invention
This invention relates to a system for processing pixels representative of the informational contents of a document. Particularly, the invention relates to an automatic system for processing pixels representative of mixed format documents containing text (line copy) material, photographic (continuous tone) material, and halftone material.
2. Description of the Prior Art
The processing of documents for electronic copying or facsimile transmission is well known in the prior art. The processing of documents includes the steps of scanning the document and generating a video stream of pixels (picture elements). The pixels are the electronic representation of the informational contents of the document. The video stream of data is next thresholded and printed.
The prior art abounds with scanners suitable for generating video streams of pixels. Generally, the pixels are generated by reflectively scanning the document. By way of example, U.S. Pat. No. 4,146,786 is representative of the prior art. In the patent, a plurality of linear sensor arrays are used to scan a line of data on an original document.
Generally, text material and continuous tone material are thresholded with different thresholders suitably optimized to take advantage of their different characteristics. Text material is optimized by a thresholding technique which results in sharp edge definition, high contrast and high frequency detail rendition. The prior art usually uses a single fixed thresholder or variations thereof to process text materials. The fixed thresholder is usually placed at the midpoint of a gray scale range. The video representation of the textural material is measured against the fixed thresholder. An example of such a thresholding technique is in U.S. Pat. No. 3,723,649. Continuous tone material is optimized by a thresholding technique which results in good gray scale reproduction and only moderate detail rendition. Examples in the prior art are the so-called "matrix thresholding" which includes a matrix of different thresholding values repeated periodically across the document and the so-called "error diffusion." The thresholding techniques which are optimized for one type of material generally yield results which are marginal to poor on the other type of material.
Attempts have been made to switch between thresholders optimized for text or continuous tone reproduction. The switching is usually based on local information surrounding the pixel of interest. Examples in the prior art are U.S. Pat. Nos. 3,294,896 and 2,255,408. Generally, these fall into two classes: switching determined by gradient or frequency and switching determined by video level.
U.S. Pat. No. 3,294,896 is an example of the latter. In the method taught by this patent, signals developed by a photocell are passed through a two-level threshold network. Signals above the upper threshold are transmitted as black and signals below the lower threshold are transmitted as white. Signals falling between the two levels are transmitted as gray and represented by the results of thresholding by a continuous tone thresholder.
U.S. Pat. No. 2,255,408 is an example of gradient or frequency switching. High frequency changes in reflectance are assigned to text thresholders and low frequency changes in reflectance are assigned to continuous tone thresholders.
Both of the above methods are inadequate for high quality reproduction of mixed format documents. The former fails in the transition regions on the edge of text and the latter in the flat middle sections of text characters. Attempts to avoid this degradation in text reproduction by narrowing the range between upper and lower thresholds result in severe truncation of the gray scale range of the continuous tone reproduction.
U.S. Pat. No. 3,622,698 describes a facsimile system which can be manually controlled depending on whether the document being scanned is text or photograph. A switch is provided for an operator to adjust based on the format of the document. The control is achieved by changing the threshold and saturation level of the transmitted signal.